Holy Guacamole!

Chcr, it's all levels of protection. Once you get past 99.995, more is just flood insurance in the Sahara. I'm a big fan of online spares. My raid 5 has at least one online spare. Costs the price of an unused drive ... right up until you need it. Hot swap drives add more flexibility. Raid 5+0 essentially doubles the number of read heads on the array. Raid 5+1 moves the number of "safe" failure points about. Y'see, in a normal raid 5, you can lose one drive, and you're still good to go. Lose two, and you're toast. Online spares mean that the instant one goes down, the spare comes online and begins the rebuilt. Lose another before that's complete ... and you're toast. 5+1 means that you can lose 2 drives, and still have a shot at rebuilding ... provided you didn't lose the mate of the first one lost.
 
I guess RAID 5+1 is for the paranoid, a more reasonable standard is RAID 6.

RAID 6 is essentially the same as RAID 5 but you have distributed dual parity, meaning that you can lose at most any 2 drives.
 
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